Domestic swimming pools have been increasing in popularity for years. This is particularly the case in the states of the Sun Belt where mild weather permits pools to be used as much as ten months of the year. All pools require chemical treatment to destroy potentially harmful bacterial and unsightly algae growths. In the past, treatment has most commonly been done by daily additions of calcium hypochlorite powder to maintain a residual chlorine level of about 1 ppm, determined by a colormetric test kit.
In recent years, sharp increases in the cost of energy have caused major increases in the cost to the consumer of calcium hypochlorite because this chemical is produced by electrical energy and because it is shipped as 65% calcium hypochlorite stabilized by 35% inert ingredient. By contrast, ordinary salt has not escalated in price and can be used in an electrochemical cell to produce chlorine at the point of use. Several manufacturers market small cells for domestic swimming pools and these have been accepted by consumers on the basis of major savings in cost of chlorinating their pools. The cells are incorporated in systems which allow chlorination to proceed automatically whenever the pump of the pool filter is operating.
In addition to cost savings compared to purchased calcium hypochlorite convenience in chlorinating their pools is a more important point with most consumers. A cell which operated with little or no maintenance, incorporated in a system which automatically chlorinates the pool and which requires the user's attention infrequently, for example once a week, will have strong chemical appeal in this market.
A further advantage of automatic chlorinator systems, is that residual chlorine levels can be maintained at uniform levels. This contrasts with manual addition of excess calcium hypochlorite once a day; in this case excess residual chlorine exists at certain times and there may be no residual chlorine at other times.
There are two approaches to chlorinating domestic swimming pools when using salt as starting material. One approach is to produce chlorine gas in a cell designed to allow removal of the gas. This proceeds by this electrochemical reaction: EQU 2 NaCl+2 H.sub.2 O.fwdarw.2 NaOH+Cl.sub.2 +H.sub.2 ( 1)
The second approach is to produce sodium hypochlorite. This is done by design and operation of a cell in which chlorine is reacted with caustic as produced. This chemical mixing reaction proceeds as follows: EQU Cl.sub.2 +2 NaOH.fwdarw.NaClO+NaCl+H.sub.2 O (2)
In industry, both processes are carried out on large scale. Equation 1 describes the process by which millions of tons of chlorine are produced annually throughout the world. Industrial chlorine plants operate continuously to produce a pure product at high current efficiency.
A domestic swimming pool requires a maximum of one pound of chlorine per day. Several cell designs of this capacity are on the market but their output of chlorine declines sharply after a short period of use. This unsatisfactory performance is due to faulty design, as can be seen from the following discussion.
It appears attractive to produce chlorine as gas in a small cell because the gas can be readily removed from the cell by aspiration through plastic tubing connected to the suction side of the swimming pool pump. With suitable wiring (timer, switch etc.) this allows chlorine to be produced and gradually fed into flowing water whenever the pump is operating. The problem is that when chlorine is produced at the anode, an equivalent amount of caustic is produced at the cathode. The two products must be physically separated to prevent the hypochlorite reaction, (Equation 2) from taking place. This separation is accomplished in swimming pool cells by placing an ion selective membrane between anolyte and catholyte compartments. In operation, salt and water are supplied to the anolyte. Under a potential sodium ions pass through the membrane to form caustic at the cathode. The membrane prevents caustic from entering the anolyte compartment, so chlorine produced at the anode soon exceeds its low solubility in brine and goes out of the electrolyte as a gas.
However, all ion selective membranes suitable for use in chlorine cells, are very sensitive to clogging by cations such as calcium and magnesium. In industrial chlorine plants which use membrane cells, calcium and magnesium are common impurities in the raw brine. They are removed by caustic and soda ash precipitations, followed by treatment of the brine with ion exchange resins and a polishing filtration step. The treated brine is continuously monitored for hardness by sensitive detectors to prevent calcium and magnesium from entering the membrane cells. Small chlorine cells for use with swimming pools are charged with salt of varying purity dissolved in water from the household supply or from the pool. Calcium and magnesium are almost always present. As these ions clog an ion selective membrane, transport of sodium ions is reduced and chlorine output drops proportionately.
When used by the general public, ion selective membranes are subject to puncture because they are thin, typically having thickness of five of six mills. Even small punctures allow caustic to enter the anolyte and this reduces evolution of gaseous chlorine. Another drawback is that ion selective membranes cost $35 to $40 per square foot at wholesale, so frequency replacement by the consumer is a substantial operating expense.
Some manufacturers of swimming pool chlorinators have designed systems in which a low level of salt is maintained in the swimming pool. When pool water is circulated through a cell, chlorine is produced electrochemically. There is no separator in the cell and chlorine is converted to sodium hypochlorite by reaction with caustic. These cells can be quite compact and are usually installed in the return line from the filter to the pool. This design approach avoids the problems of membrane cells, but new problems are introduced and frequent failures occur. A discussion of these problems follows.
If the salt supply for a cell comes from the swimming pool, salt content must be very low to avoid discomfort to swimmers. Chlorinator manufacturers recommend salt content of 0.3%, about one tenth the content of sea water. An electrolyte of this concentration has high electrical resistance and cell designers have compensated for this by spacing electrodes closely, typically one sixteenth of an inch. In these designs, anodes are made of titanium sheet or mesh coated with an electrocatalyst such as ruthenium oxide or platinum. Cathodes are titanium or steel and electrode spacing of one sixteenth is readily achieved. Most swimming pools contain calcium in solution, derived from the water supply, prior treatment with calcium hypochlorite or from leaching of concrete structures. In a cell, calcium compounds are formed as tenacious deposits on the cathode. In a few weeks of operation, these deposits can clog a sixteenth inch space resulting in reduction of electrolyte flow through the cell and finally in clogging. When electrolyte flow is greatly diminished, the cell can be destroyed by overheating. This has been a common type of failure of one cell on the market. This cell uses more than 200 Watts, supplied to a volume of about half a liter.
Designers have approached the clogging problem in hypochlorite cells by interrupting the voltage periodically, for example, for 10 minutes in each hour of operation, to allow fluid flow to remove cathode deposits. This is not a complete solution, as field failures continue to be common. It is very well known in the art that these deposits are not easily removed. One manufacturer recommends that the user remove the cell periodically and soak it in hydrochloric acid to dissolve calcium deposits. This raises problems of safety as the consumer must transport, store and use a strong acid.
In systems which operate on a low salt content maintained in the pool, there is another cause for premature failure of the cell. In a situation where flow is significantly reduced, salt content can be completely depleted and oxygen becomes the product at the anode. Anode coatings such as ruthenium oxide or platinum have very long lives when evolving chlorine. When evolving oxygen, ruthenium oxide wears rapidly and passivates quickly. Platinum wears less rapidly than ruthenium oxide, but still at a rate many times the rate when evolving chlorine. In domestic swimming pools, reductions in flow commonly occur due, for example, to an article of clothing drawn into the pump strainer or to a filter which is greatly overloaded while operating unattended.
To summarize the prior art in the field of domestic swimming pool chlorinators, there have been two design approaches to cells and systems. One method is to produce chlorine gas and mix this into recirculating pool water. The other method is to produce sodium hypochlorite from a low salt content in the pool water. Three or four manufacturers market systems to swimming pool owners. Despite variations in designs, all systems are subject to early and frequent cell failure. There is a need for a cell which will operate reliably when used by the general public under typical conditions found in domestic swimming pools. There is a further need that a reliable chlorinator cell be incorporated in a system which can automatically feed cell product into a swimming pool.